The present invention concerns a test apparatus for providing axial stresses in a structure. More particularly, but not exclusively, this invention concerns a test apparatus for providing axial stresses in a structure, the structure having a first surface on one side and a second surface on an opposite side. The invention also concerns a method of providing axial stresses in a structure.
An existing test arrangement is shown in FIG. 1. The test arrangement 1 is testing a sandwich structure 10. The sandwich structure 10 has a top surface 11 and a bottom surface 12. The structure 10 is made up of a top skin 13, bottom skin 14 and a core 15. The test apparatus comprises a base structure 20, a top structure 30 and a load block 40.
The base structure 20 comprises a base platform 21 from which two upwardly extending formations 22, 23 extend. The formations 22, 23 are curved at their tip to minimise the lengthwise distance along which the load is applied. The formations 22, 23 extend along the width of the structure 10. On top of each formation 22, 23 grease 26 is applied to aid rotation, if large downward deflections of the structure 10 are expected. Then a Vee block 25 and then a rubber pad 26 are placed on top of the formations 22, 23. The Vee block 25 and rubber pad 26 reduce the through thickness stress exerted on the structure 10 at the formations 22, 23. The sandwich structure 10 is placed on top of the two rubber blocks 24.
The top structure 30 comprises a top platform 31 from which two downwardly extending formations 32, 33 extend. The formations 32, 33 are curved at their tip to minimise the lengthwise distance along which the load is applied. The formations 32, 33 extend along the width of the structure 10. On the end of each top formation 32, 33 grease 36 is applied to aid rotation, if large downward deflections of the structure 10 are expected. Then a Vee block 35 and then a rubber pad 36 are placed on the end of the formations 32, 33. The Vee block 35 and rubber pad 36 reduce the through thickness stress exerted on the structure 10 at the formations 32, 33. The top structure 30 is placed on top of the sandwich structure 10 so that the rubber pads 34 sit on the top surface 11 of the structure 10.
In use, the sandwich structure 10 is placed in between the upwardly extending and downwardly extending formations 22, 23, 32, 33 and a load block 40 is placed on the top of the top platform 31. This causes the structure 10 to bend downwards at all locations between the two upwardly extending formations 22, 23. Maximum downward bending occurs between the two downwardly extending formations 32, 33. Relative upward bending occurs at the outer portions of the structure 10 (either side of the two upwardly extending formations 22, 23). This creates uni-axial stresses (along a single axis) in the portion of the structure between the two downwardly extending formations 32, 33 and allows this portion to be tested. The maximum downwards deflection can be measured at the mid-point between the two downwardly extending formations 32, 33.
This test apparatus is normally used for thin skinned sandwich panel structures. This is because in these structures, the test produces near uniform uni-axial compressive stress on the top surface 11 and near uniform uni-axial tensile stress in the bottom surface 12. The bending moment and skin stresses are maximum and constant in between the two downwardly extending formations 32, 33. Through thickness shear forces are only produced on either side of the downwardly extending formations 32, 33. The test apparatus is normally used for sandwich structures with a lightweight (for example, honeycomb) core and thin composite (for example, fibre reinforced plastic or carbon prepreg) skins. In particular, the test works best if the core depth is over 6 times the skin thickness.
The test is used to investigate the strength of the sandwich skins. Therefore, ideally the load used and the formation positions are designed to ensure skin failure (prior to core shear failure).
The test rig is simple and can be used by using only a single downward load 40.
However, it is often desired to test a structure by applying bi-axial loads. Currently, this can be done by attaching an actuator to each of four different legs of a cruciform structure, and applying a pulling or pushing force to each actuator. However, these bi-axial rigs require a complex calibration to confirm the biaxial loads in the structure compared to the forces applied at the actuators. This is because, the actuator on one leg, even if no force is applied to it, constrains the leg and affects the stress in the structure due to Poisson deformation being prevented. This makes bi-axial testing complicated, costly and time-consuming.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved test apparatus for and method of providing bi-axial stresses in a structure.